Method of regenerating a spent photographic silver bleach solution

ABSTRACT

In a spent photographic silver bleach solution the ratio of ferrocyanide to ferricyanide has attained a value at which the efficacy of bleaching is so greatly impaired that the solution must be either discarded or regenerated. According to the present disclosure, regeneration is accomplished with an alkali metal halite that reacts with the ferrocyanide to oxidize it to a ferricyanide. The reaction takes place at a pH between about 6 and about 8. An acid preferably is present to prevent the reaction conditions from becoming so alkaline as to materially inhibit the oxidation.

United States Patent 1 Bulloch et a1.

[ Oct. 14, 1975 METHOD OF REGENERATING A SPENT PHOTOGRAPHIC SILVERBLEACH SOLUTION [73] Assignee: Philip A. Hunt Chemical Corporation,Palisades Park, NJ.

22 Filed: May 21,1973

211 Appl. No.: 362,243

[52] US. Cl 96/60 R; 96/50 A; 423/367 [51] Int. Cl. G03C 5/32; COlC 3/12[58] Field of Search 96/60 R, 50 A; 423/367,

[56] References Cited UNITED STATES PATENTS 2,515,930 7/1950 Seary 96/50A 2,611,699 9 1952 Zappert 96/50 A 2,611,700 9/1952 Brunner 96/50 A3,770,437 1 H1973 Brugger et al. 96/60 R OTHER PUBLICATIONS Def. Pub.T878,007, Walsh, 9-8-1970. Treatise on Inorg. Chem. by H. Remy. Vol. 1,1956, pp. 810-81 1.

Primary Examiner-Mary F. Kelley Attorney, Agent, or Firm-Kirschstein,Kirschstein, Ottinger & Frank [5 7] ABSTRACT In a spent photographicsilver bleach solution the ratio of ferrocyanide to ferricyanide hasattained a value at which the efficacy of bleaching is so greatlyimpaired that the solution must be either discarded or regenerated.According to the present disclosure, regeneration is accomplished withan alkali metal halite that reacts with the ferrocyanide to oxidize itto a ferricyanide. The reaction takes place at a pH between about 6 andabout 8. An acid preferably is present to prevent the reactionconditions from becoming so alkaline as to materially inhibit theoxidation.

8 Claims, No Drawings METHOD OF REGENERATING A SPENT PHOTOGRAPIIICSILVER BLEACH SOLUTION BACKGROUND OF THE INVENTION 1. Field of theInvention Regeneration of a spent photographic silver bleach solution.

1. Description of the Prior Art Bleach solutions are widely used incertain branches of photography. They are employed to remove metallicsilver grains from an emulsion in which they were formed by developmentof exposed particles of silver halide. Typically, the bleaching isperformed in color film processes to remove metallic silver grains thepresence of which is necessary to permit the formation of colored dyeimages, and in reversal black and white processes to remove the silvergrains which formed the negative image prior to the. re-exposure of filmfor actinically sensitizing the remaining particles of silver halide.The purpose of bleaching is to convert the metallic silver into a silversalt which thereafter is made water soluble by fixation (treatment withthiosulfate) and subsequently removed from the film by washing withwater. Although the conversion of metallic silver to the silver saltoccurs in several steps the particular reaction is not important to thediscussion of this invention and, therefore, only the overall simplifiedionic equation is given below:

A commonly and widely used bleach solution contains as the oxidizingagent an alkali metal ferricyanide and an alkali metal bromide to effectthe reaction above given, and usually other ingredients such as bleachaccelerators (see, for example, British Pat. No. 1,150,466), buffers(see, for example, U.S. Pat. No. 3,342,598), corrosion inhibitors (see,for example, British Pat. 872,275), anti-sludge agents, and biocides(see, for example, British Pat. 1,1 77,048). The particular constitutionof the bleach solution is of no particular importance to the presentinvention as long as it contains ferricyanide which is in part reducedto ferrocyanide during the bleaching process. As just implied, theferricyanide oxidizes metallic silver to silver in its ionic form whichthereupon reacts with the bromide ion to form the sparsely solublesilver bromide. As the ferricyanide oxidizes the metallic silver, theferricyanide itself is reduced to ferrocyanide.

Ferrocyanide accumulates in the bleach solution. The decrease inferricyanide and the corresponding increase in ferrocyanide graduallyimpairs the efficacy of the bleach until finally a point is reachedwhere the solution no longer is commercially effective and either mustbe discarded or it must be regenerated.

At this point there still is a preponderance of ferricyanide. It is onlythe ferrocyanide which must be regenerated by converting the same backto ferricyanide.

The actual ratio between the two cyanides has no bearing upon the use ofthe present invention and, indeed, the limiting ratio beyond which thesolution should not be used will depend upon various other factors such,for example, as the solution temperature and the bleach acceleratorsemployed. Mention has been made of the spending of ferricyanide simplyin order to understand the manner in which the ferrocyanide came intobeing and the necessity of restoring ferrocyanide to ferricyanide whenthe occasion requires. Such necessity for regeneration long has beenrecognized in the photographic art and the specific regeneration offerrocyanide to ferricyanide has been practiced for many years.

At one time, an electrolytic method was employed for this purpose, butbecause of its high maintenance costs and operational problems it wasabandoned.

A number of chemical agents have been proposed to oxidize ferrocyanideto ferricyanide. Thus, liquid bromine is disclosed for this purpose inU.S. Pat. No. 2,515,930. But bromineis caustic and corrosive and, hence,great caution must be exercised when it is employed. As a variant of thebromine oxidation it has been proposed to use bromine releasing agentsas, for example, those disclosed in U.S Pat. No. 2,61 1,700 for thispurpose. Also, the use of hypobromides or mixtures of bromate ions andhydrobromic acid have been disclosed as in U.S. Pat. No. 2,611,699. Noneof these methods has found wide application, either because of thecaustic and corrosive nature of the compounds or the bromine released orbecause of their high cost or because of the involved manipulationrequired.

Recently ozone oxidation has come into commercial usage (see T. W. Boberand T. J. The Regeneration of Ferricyanide Bleach Using Ozone Parts 1and 11, Image Technology, Vol. 14, No. 4- -p. 13; No. 5p. 19 [1972]).The article describes large scale regeneration trials on Process CRl-land Process K-12 bleaches using ozone, and suppliers of completefactory-built systems for bleach regeneration are given. However, thereis a difficulty with the ozone oxidation in that the equipment used forgeneration of ozone has a high initial capital cost and a high operationcost. Moreover, generation of ozone is inefficient in terms of energyconversion. Still further, the formulations for ozone generation arerapidly being changed so that often generating equipment is obsoletedbefore its initial cost can be fully amortized.

Another approach to the regeneration of ferrocyanides has been by theuse of peroxides and per-acids, but in such cases the bleach must bemade very acidic for the reaction to proceed rapidly and to completion;The high acidity results in undesirable side reactions and undulycomplicates the regeneration of ferricyanide.

At the present time the most widely practiced method for convertingferrocyanide to ferricyanide utilizes a water-soluble persulfate as theregenerating agent. The latter method has several advantages. Capitaloutlay is low, the method is relatively safe and it is simple to use.However, it suffers several drawbacks, the most serious of which is theformation of sulfate ions as a by-product of regeneration. These ionsreduce the efficacy of the bleaching action of the ferricyanide onmetallic silver grains, the reduction being so serious that after fouror five regeneration cycles the bleach has to be discarded. It is notcurrently feasible to inexpensively remove the sulfate ions so as torestore the tant consequence of a high sulfate concentration in thebleach is the necessity for a higher equilibrium concentration offerricyanide than would be necessary if sulfate ions were not present. Acursory comparison of sulfate-free with sulfate-containing bleachesindicates that the ferricyanide concentration for a sulfate-ionfreebleach for an equivalent rate of bleaching is about 30% less than for ableach containing a saturation quantity of sulfate ions.

Also involved in connection with ferricyanide silver bleaches and inwhich the spent bleaches contain ferrocyanide as well, and which, forexample, with the persulfate regenerating process require discarding ofbleach fractions to keep the sulfate ion concentration at an acceptablelevel, is that economic and conve nience factors are not the onlygoverning considerations as they once were. In the present day, thedemands of ecology require that pollution be kept to a minimum.Ferricyanide and ferrocyanide can decornpose to form cyanide when suchsolutions are dumped either into a sewer or a running or quiescent bodyof water. Due to the large concentration of these ions in bleacheffluent, maximum allowable cyanide concen trations which represent thelimits for the sewer codes in many municipalities are exceeded far tooeasily, so that given the present state of affairs, pollution abatementhas become the deciding factor and regeneration is often the onlyacceptable alternative.

SUMMARY OF THE INVENTION 1. Purposes of the Invention It isan object ofthis invention to provide for regenerating spent photographic silverbleaching solutions a method which avoids all the foregoing drawbacks.

It is another object of the invention to provide a method of thecharacter described which will regener' ate spentferricyanide/ferrocyanide-containing photographic bleaching solutionswith a low capital outlay and low maintenance costs, and with an abilityto be operated by personnel of low skill.

It is another object of the invention to provide a.

method of the character described which can be practiced economicallyand safely and, withal, is rapid and efficient.

Other objects of the invention in part will be obvious and in part willbe pointed out hereinafter.

2. Brief Description of the Invention In accordance with the presentinvention, a spent photographic silver bleach solution is regenerated byreacting such solution with an alkali metal halite to oxidizeferrocyanide ions to ferricyanide ions. The desired halite is a bromite.The reaction proceeds best at a pH in the neighborhood of neutrality.More specifically, it will proceed rapidly and apparentlystoichiometrically between a pH of about 6 and a pH of about 8. It hasbeen observed that at a pH of above 8.5 the regenerating reactionapproaches a standstill.

The oxidation reaction of the halite on the ferrocyanide ions generateshydroxyl ions which tend to raise the pH of the solutions beyond theacceptable range. Moreover, sodium bromite is not considered stableenough to store in dry form. It can decompose violently on contact withorganic matter and acids, so that for safe handling it is customary toship it diluted in a dilute aqueous solution of sodium hydroxide, forinstance, at 3.5% sodium hydroxide water solution containing about 6 to8% of sodium bromite. This, too, therefore. is strongly alkaline andtends to raise further the pH of the spent photographic silver bleachsolution being regenerated. To prevent these two alkalizing effects. itis desirable to compensate for the rise in pH by including an acid inthe regenerating reaction. Any acid is useful for example, a 1071 watersolution of an orthophosphoric acid. A preferred acid is hydrobromicacid be cause it does not add any extraneous ions to the system and,indeed, provides a bromide ion which is useful in the system. I

It is of passing interest to observe that the same reaction, whichessentially is an oxidation of the ferrocyanide constituent of the spentphotographic silver bleaching can be used for the manufacture of analkali metal ferricyanide starting from an alkali metal ferrocyanide, oras a method of preparing a fresh ferricyanide bleach starting with acomposition including an alkali metal ferrocyanide as all or part of theamount of cyanide eventually desired to be present as a ferricyanide. I

The invention consists in the series of steps which will be exemplifiedin the method hereinafter described and of which the scope ofapplication will be indicated in the appended claims.

PREFERRED EMBODIMENT OF THE INVENTION A spent photographic bleachsolution, this being a solution in which the amount of ferrocyanide isgreat enough, as is well known, to reduce the rate of bleaching to anunacceptable level, is regenerated in accor-.

dance with the present invention by treating such a solution with awater solution containing an alkali metal halite, preferably in thepresence of an acid, desirably a mineral acid, and most desirablyhydrobromic acid so as not to unnecessarily introduce extraneous ionsinto the bleaching solution. The regenerated bleach formed issubstantially equivalent to the fresh bleach of a like composition. Byselecting a preferred halite, to wit, a bromite, and a preferred acid,undesirable foreign ions are not added to the bleach solution, minimumtime is required for completion of the regeneration, the regeneration isrelatively safe and inexpensive and the method virtually eliminatesbleach overflow disposal problems. The presumed overall ionic reactionis:

BrOf 4Fe [CN] 2H O 4Fe [CN] +Br' Experimental evidence suggests atwo-step reaction expressed ionically as follows:

The conversion of ferrocyanide to ferricyanide proceeds quite rapidlyand apparently stoichiometrically between a pH of about 6 and a pH ofabout 8.

To practice the conversion of an alkali metal ferrocyanide to an alkalimetal ferricyanide, pursuant to the instant invention there is added toan aqueous ferrocyanide solution a stoichiometric amount of bromite, forexample, in the form of sodium bromite. As indicated previously, thesodium bromite which is a very strong oxidizing agent is notsufficiently stable to be safely stored dry and, therefore, is suppliedcommercially as an alkalized dilute water solution of sodium bromite,for example, a 6 to 8% sodium bromite water solution containing 3.5%sodium hydroxide. Such a solution is commercially available from OlinCorporation under the trademark Preptone. Where such a stoichiometricamount of bromi'te is added to an unbuffered ferrocyanide solution andthe pH is adjusted to neutrality with hydrobromic acid, the conversionof ferrocyanide to ferricyanide is essentially complete within aboutminutes. It has been observed, however, that the pH of the regeneratedsolution rises and continues to rise for up to about 6 hours, suggestingthat the first-mentioned reaction of the above twostage reaction israpid and the second one is slow.

The invention is practiced in connection with a photographicferrocyanide/ferricyanide spent bleach solution by adding to such aspent solution wherein the amount of ferrocyanide has previously beenascertained a stoichiometric amount of bromite, again in the form ofsodium bromite, e.g. supplied as Preptone, and again adjusting the pH tobetween about 6 to about 8 with hydrobromic acid. The amount ofhydrobromic acid employed will depend upon the specific composition ofthe particular bleach solution and the degree of its exhaustion.

Where a buffered bleach is regenerated the pH rise is greatly depressed,that is to say, does not rise as rapidly as where the method ispracticed with an unbuffered ferrocyanide solution, and the recorded pHchanges usually are complete within approximately 30 minutes. Theconversion of the ferrocyanide ions to ferricyanide ions is completewithin 10 minutes or less.

The invention may be used with any of the conventional ferricyanidebleach solutions that commonly are used for photographic processingsuch, for example, as those employed in color film processing for thebleaching step and those employed in reversal black and white processingfor the bleaching step. These bleaches typically include aqueoussolutions containing an alkali metal ferricyanide such as potassiumferricyanide (K Fe [CN] sodium ferricyanide Na, Fe [CN]6), lithiumferricyanide (Li Fe [CN] or ammonium ferricyanide ([NH Fe [CN] and analkali metal bromide. Mixtures of these ferricyanide salts also may beused.

As mentioned above, the alkaline halite salts usable in the instantregeneration process are not available or safe in dry form but they arecommercially available as aqueous solutions thereof, usually diluteaqueous solutions, typical solutions having from about 6% to about 8% ofthe halite salt dissolved therein. Such is the case with sodium bromitewhich is the desired halite of this invention. Moreover, for the purposeof maximizing the chemical stability of alkali metal and sodium halites,it is customary to make such solutions very alkaline, e.g. with sodiumhydroxide, a typical amount of sodium hydroxide employed being 3%% byweight. Such commercially available halite aqueous solutions are quitesatisfactory for use in connection with the method of the presentinvention.

The excess alkalinity of the available commercial solutions, as well asthe alkalinity produced during the course of the reaction, which tend toraise the pH of the reacting solutions to above 8 this being in excessof the range at which the change from ferrocyanide to ferricyanideproceeds rapidly, can be and, pursuant to the present invention, isoffset with either a mineral acid or an organic acid or an acidbuffering agent. Inasmuch as conventional ferricyanide bleaches normallycontain buffering agentsto control the pH of the bleach, the selectionof the neutralizing acid should be done bearing in mind the quantity andconstitution of the buffering agent which is present in the bleach beingregenerated. For example, if the bleach being regenerated contains aphosphate buffer system therein, the preferred acid utilized in theregenerating system of the present invention for the neutralization ofexcess alkalinity desirably would be orthophosphoric acid. Anotherdesirable acid use for the foregoing purpose is hydrobromic acid.

The following example has been set forth to aid in understanding the useof the present invention.

A typical standard photographic bleach for the bleaching of colorreversal film has the following formula:

Potassium ferricyanide 56 g Potassium bromide [65 g Sodium phosphate 22g Water to one liter pH 6.90

This bleach was used to replace the conventional bleach used in theEktachrome E-3 process. The above formula gave photographicsensitometric results equivalent to those employed with the replacedformula. The film used was Ektachrome sheet film. The aforesaid filmafter having been exposed in a camera was processed at a rate of 2square feet per gallon per process cycle using conventional solutionsthat are employed in the Ektachrome E-3 process except for the bleachsolution which was replaced, as just indicated, with the above solution.All other solutions were replenished in accordance with Eastman Kodakinstructions which are standard and well known in the art.

After eight process cycles, which is the equivalent of approximately 17square feet of film per gallon of the substituted bleach, the bleach wasanalyzed and thereafter regenerated with the aforesaid sodium bromitesolution (Preptone which has been described earlier). A certain amountof dilution occurred from wash water carried over from the processingstep previous to the bleach step; such dilution losses were made upbefore or after each regeneration by adding potassium ferricyanide andpotassium bromide and the pH was adjusted during each regeneration withorthophosphoric acid.

The aforesaid sequence was repeated until 47 square feet of film pergallon of bleach had been processed; this, of course, includedintermediate regeneration steps with sodium bromite solution.

The following table is a summary of the chemical composition of thebleach at the various processing stages:

The pH of the bleach was adjusted to 6.9 i 0.1 during and after eachregeneration. Sensitometric gray-scale control strips were processedwith each process cycle and. other than normal run-to-run variability,no significant density differences occurred during the course of thetest. The dye maximum density area of the control strips were also readon a Westrex RA 1100B sound densitometer and no significant differencesin density readings appeared over the duration of the test. Chemically,the bleach at the end of the regeneration series was essentially thesame as at the beginning of processing with no build-up of interferringforeign ions.

The quantitative details of the aforesaid example included firstassaying the sodium bromite solution.

Commercial sodium bromite solutions contain 60-80 grams per liter ofactive ingredient and 32-34 grams per liter of sodium hydroxide; thecarrier is water. These solutions are moderately stable but should beanalyzed for bromite prior to use because some decomposition may occur.

Materials used to analyze for bromite are: a water solution of a reagentgrade potassium iodide prepared by dissolving 100 grams of potassiumiodide in water and diluting with water to one liter (the water iseither distilled water or deionized water); a reagent grade of aphosphate buffer prepared by dissolving 300 grams of crystallinemonobasic sodium phosphate (NaH PO H O) in water and diluting with waterto one liter, the water again being distilled or deionized; and areagent grade of 0.1000N aqueous solution of sodium thiosulfate.

The method of sodium bromite analysis constitutes pipetting 1 ml. ofsodium bromite solution into a 250 ml. Erlenmeyer flask containing 100ml. of water which is distilled or deionized and adding thereto 20 ml.of the potassium iodide solution and ml. of the phosphate buffersolution. Next, the mixture is titrated with the 0.1000N sodiumthiosulfate solution until the mixture is lemon yellow. Finally, a dropor two or a reagent grade soluble starch indicator is added and themixture is titrated to a colorless endpoint.

The general equation employed for calculating for sodium bromite contentis as follows:

g/liter NaBrQ,

[ml Na,S,O,] S,O,] [Equivalent wt. of NaBro,] volume of sampleParticularizing the above general equation to the aforesaid parameters,the sodium bromite assay in grams per liter equals:

It also is necessary in carrying out the present invention to assay thespent bleach for ferrocyanide and ferricyanide content beforeregenerating the same. This may be done spectrophotometrically by thefollowing method.

The equipment and reagents used are: a Beckman DB spectrophotometer oran equivalent device equipped with 10 mm. cuvettes; a reagent grade 2%sodium bicarbonate solution prepared by dissolving 20 g. of sodiumbicarbonate in distilled or deionized water and diluting with distilledor deionized water to one liter; a reagent grade potassium ferricyanidesolution prepared by diluting 0.2 g. of potassium ferricyanide (i 0.1mg) to one liter in a 27: sodium bicarbonate solution of distilled ordeionized water; and a reagent grade potassium persulfate solutionprepared by diluting 1.2 g. of potassium persulfate to one liter with a2% sodium bicarbonate solution of distilled or deionized water.

The method for analyzing for potassium ferricyanide constitutes dilutingthe spent bleach sample in two steps, the first of which consists inpipetting 3 ml. of the sample into a flask and diluting to ml. with the2% sodium bicarbonate solution; the second step consists in pipetting 5ml. of the resulting dilute solution into a flask and diluting a secondtime to 100 ml. with the 271 sodium bicarbonate solution. The absorbanceof the twice-diluted spent bleach sample is read on thespectrophotometer at 420 nm. using a 10 mm. cuvette and using the 2%sodium bicarbonate solution as a blank. The absorbance of the standardpotassium ferricyanide solution is read in a similar manner, i.e. afterdouble dilution as aforesaid.

The calculation for potassium ferricyanide employs the followingequation:

Potassium ferricyanide [grams/liter] sample [g/l K;,Fe[CN], in standardsolution] [10,000] absorbance standard absorbance 15 the potassiumpersulfate solution. The bottle is closed with a glass stopper, shakenwell and allowed to stand at 2025C for one hour, whereupon 10 ml. of thereaction mixture is pipetted into a flask and diluted to 100 ml. withthe 2% sodium bicarbonate solution. Now the absorbance of the reactionmixture is read on the spectrophotometer at 420 nm. using a 10 mm.cuvette and using the 2% sodium bicarbonate solution as a blank.

The absorbance of a similarly twice-diluted and similarly treatedstandard potassium ferricyanide solution is read in the same fashion.

After having determined the potassium ferricyanide content of the bleachby the previous method, the total ferricyanide present after fullconversion of the ferrocyanide is obtained by the following formula:

To determine the sodium ferrocyanide decahydrate content, the followingformula is employed:

grams/liter sodium ferrocyanide decahydrate total potassium initialpotassium] 1 ferricyanide ferricyanide [g/] sodium ferrocyanidedecahydrate [gll] 0.0696

V l.b t

mm! c bromite assay [g/l] [liters] Next in the practice of the inventionthe quantity of sodium bromite solution required to oxidize all theferrocyanide to ferricyanide, e.g. calculated in the foregoing manner isadded to the spent bleach solution and the pH of the solution in whichthe ferrocyanide is being oxidized is adjusted to neutrality with anacid, e.g. an orthophosphoric acid or hydrobromic acid. The acidemployed preferably is added in a water solution and may be quiteconcentrated. For example, when using hydrobromic acid a 48% aqueoussolution thereof is employed. The neutralizing acid is not premixed withthe alkaline sodium bromite solution.

The bleach is now regenerated and as a check for efficient reuse theregenerated bleach is assayed for each of its constituents such, forexample, as ferricyanide, bromide and other addenda normally present ina fresh bleach. Standard analytical assay methods are employed. It willbe recalled that during the bleaching method the bleach solution in thebleaching tank is diluted by carry-over of solution from the precedingtank and it is this which necessitates the aforesaid assay of theconstituents of the regenerated bleach.

Finally, additional compounds are added to make up the differencescaused by dilution or use, the added compounds being solid or watersolutions of potassium ferricyanide, sodium bromide and other addenda asare required.

The foregoing description of the method of the invention provides allthe instructions necessary to carry out said method. Nevertheless, tocomplete the description, there is set forth below an example of aquantitatively described series of steps for regenerating a spentferricyanide bleach solution, i.e. one containing some ferrocyanide:

To 440 milliliters of an exhausted ferricyanide bleach, found byanalysis to contain l0l grams per liter of sodium ferricyanide, Na -,Fe[CN] .H,O, and 84 grams per liter of sodium ferrocyanide, Na Fe[CN]',.10H,O [47 grams per liter of sodium ferrocyanide, Na Fe[CN],.10H,O, were present and 37 grams per liter of sodium ferrocyanide,Na Fe [N] .l0H' O,

were added], and a pH of 7.86, there were added 36 ml. (a stoichiometricquantity) of a 7'71 solution of sodium bromite, pH 13 to 14 (Preptone)and 24 milliliters of 4N hydrobromic acid. The sodium ferrocyanide,Na,Fe [CN] .lOl-l O. was added to raise the eventual concentration offerricyanide in the exhausted bleach to the concentration normally foundin a fresh bleach. After one hour at room temperature the bleachsolution was analyzed and found to have a volume of 500 milliliters andcontain 132 grams per liter of sodium ferricyanide, Na Fe [CN] .H O, ascompared to 133 grams per liter of sodium ferricyanide, Na Fe [CN] .H O,if all the ferrocyanide in the exhausted bleach had been converted toferricyanide. The pH of the regenerated solution was 7.92 as compared toa pH of 8.0 for a fresh bleach.

The step of halitizing as described in detail above can be employed toproduce an alkaline ferricyanide from an alkaline ferrocyanide as amethod for producing sodium ferricyanide apart from a bleach, and thesame halitizing step can be used to prepare a ferricyanide bleachstarting from a ferrocyanide base, i.e. a bleach in which the cyanide ispresent principally as a ferrocy anide and is converted to aferricyanide for commercial use of the bleach.

Certain bleach addenda react with alkali halites. For example, sodiumthiocyanate is completely destroyed by reaction upon the addition of abromite to a bleach containing the same. Hence, this compound must beeliminated from any bleach formula regenerated by the technique of thepresent invention. .Carbowax 1,540 (a polyethylene glycol) is notnoticeably affected by sodium bromite. It has been observed that when asodium bromite solution is mixed with a 50% Carbowax l,540 solution fora week, there is some slight oxidation of the Carbowax; but sine this isnot a procedure which would be followed with the use of the presentinvention, it is not necessary to employ bleach formulations withCarbowax eliminated. I

The use of the invention has not appeared to cause any dye degradationon color film which has been bleached by a regenerated bleach solution.

The present invention also is useful to regenerate potassiumferricyanide bleaches such, for instance, as an ME-4 type offerricyanide bleach. An ME-4 ferricyanide bleach replenisher and freshbleach usually contains about g/l of potassium ferricyanide. The average concentration of potassium ferricyanide present in a tank of spentbleach following the ME-4 process is l 12 g/l and the equilibrium tankconcentration of ferrocyanide is about 30 g/l as potential potassiumferricyanide. In the practice of the invention enough bromite is addedto convert about two-thirds of the ferrocyanide to ferricyanide, to wit,about 20 g/ l and about 30 g/l of dry potassium ferricyanide is added toraise the total potassium ferricyanide concentration after regenerationto the desired 160 g/l level.

The cost of using a halite such as sodium bromite for regeneration of aspent ferricyanide photographic silver bleach solution is higher thanthat for regeneration with a persulfate solution, but the additionalcost is largely negated by eliminating the need for discarding a portionof bleach solution when persulfate regeneration is utilized in order toprevent a build-up of sulfate. Moreover, by using a halite forregeneration ferricyanide losses to the environment can be minimized andsubstantially eliminated because the system is amenable to closedloopoperation.

It thus will be seen that there is provided a method which achieves thevarious objects of the invention and which is well adapted to meet theconditions of practical use.

As various possible embodiments might be made of the above invention,and as various changes might be made in the embodiment above set forth,it is to be understood that all matter herein described is to beinterpreted as illustrative and not in a limiting sense.

Having thus described the invention there is claimed as new and desiredto be secured by Letters Patent:

1. A method of regenerating a photographic aqueous spent silver bleachsolution containing an alkali ferrocyanide, said method comprisingadding to the silver bleach solution an aqueous solution containing analkali metal bromite to oxidize ferrocyanide ions to ferricyanide ions,the oxidation being carried out at a pH of between about 6 and about 8at room temperature.

2. A method as set forth in claim 1 wherein the alkali metal is sodium.

3. A method as set forth in claim 1 wherein the pH is maintained in therange of between about 6 and about 8 by the addition to the spent silverbleach solution in which the ferrocyanide is being oxidized of an acidselected from the group consisting of orthophosphoric acid andhydrobromic acid.

4. A method as set forth in claim 3 wherein the alkali metal bromite isin water solution and wherein said water solution also contains analkalizing agent.

5. A method as set forth in claim 4 wherein the alkalizing agent is analkali metal hydroxide.

6. A method as set forth in claim 5 wherein the alkali metal hydroxideis sodium hydroxide.

7. A method as set forth in claim 6 wherein the p of the alkaline watersolution of the alkali metal bromite is at least 13.

8. A method as set forth in claim 1 wherein the amount of alkali metalbromite added to the spent silver bleach solution is about equal to thatnecessary to stoichiometrically react with all the ferrocyanide ions toconvert them to ferricyanide ions.

1. A METHOD OF REGENERATING A PHOTOGRAPHIC AQUEOUS SPENT SILVER BLEACHSOLUTION CONTAINING AN ALKALI FERROCYANIDE, SAID METHOD COMPRISINGADDING TO THE SILVER BLEACH SOLUTION AN AQUEOUS SOLUTION CONTAINING ANALKALI METAL BROMITE TO OXIDIZE FERROCYANIDE IONS TO FERRICYANIDE IONS,THE OXIDATION BEINGG CARRIED OUT AT A PH OF BETWEEN ABOUT 6 AND ABOUT 8AT ROOM TEMPERATURE.
 2. A method as set forth in claim 1 wherein thealkali metal is sodium.
 3. A method as set forth in claim 1 wherein thepH is maintained in the range of between about 6 and about 8 by theaddition to the spent silver bleach solution in which the ferrocyanideis being oxidized of an acid selected from the group consisting oforthophosphoric acid and hydrobromic acid.
 4. A method as set forth inclaim 3 wherein the alkali metal bromite is in water solution andwherein said water solution also contains an alkalizing agent.
 5. Amethod as set forth in claim 4 wherein the alkalizing agent is an alkalimetal hydroxide.
 6. A method as set forth in claim 5 wherein the alkalimetal hydroxide is sodium hydroxide.
 7. A method as set forth in claim 6wherein the pH of the alkaline water solution of the alkali metalbromite is at least
 13. 8. A method as set forth in claim 1 wherein theamount of alkali metal bromite added to the spent silver bleach solutionis about equal to that necessary to stoichiometrically react with allthe ferrocyanide ions to convert them to ferricyanide ions.